1. Field of the Invention
The invention relates generally to the control of electromagnetic rotating machines such as permanent magnet, switched reluctance and hybrid machines thereof, and more particularly, to adaptive, smooth torque control of such machines.
2. Description of Related Art
Many electromagnetic machines in general, and electric motors employing permanent magnets in particular, exhibit torque irregularities as the rotor rotates with respect to the stator, the coils of which are typically energized with a sinusoidal waveform. Such irregularities are referred to as xe2x80x9ctorque ripple.xe2x80x9d These torque irregularities may be caused by the physical construction of a given machine. For example, they may result from the use of bearings to support the rotor. In addition, because of the electromagnetic characteristics of machines that employ magnets, the rotor tends to prefer certain angular positions with respect to the stator. Torque irregularities resulting from the electromagnetic characteristics of an electromagnetic machine are commonly known as xe2x80x9ccoggingxe2x80x9d irregularities and the resultant non-uniform rotation of the rotor or non-uniform torque output is known as xe2x80x9ccogging.xe2x80x9d Cogging is either current independent or current dependent. The first component is noted when the machine is spun unenergized. The second component is present when current flowsxe2x80x94the cogging grows as the magnitude of the stator currents increases.
In rotating electromagnetic machines employing permanent magnets, cogging most often results from the physical construction of the machine. Irregularities due to the magnets can result, for example, from the magnets being incorrectly placed upon or in the rotor, or from some irregularity about how the magnets are energized. Moreover, the utilization of rotors having discrete north and south outer poles results in a circumferential distribution of magnetic flux about the rotor circumference that is not smooth, but choppy. Additionally, the stators commonly used with such machines are formed in such a way that the magnetic fluxes generated by the stator windings provide a flux distribution about the stator circumference that is not smooth. The combination of such rotors and stators, and the accompanying non-smooth flux distributions, produces undesired irregularities in the torque output of such machines. Rotor output non-uniformities may also be produced by back emf harmonics produced in certain machines.
Obtaining smooth torque is further complicated by other factors. For instance, manufacturing variances between motors makes it difficult, if not impossible, to apply a common solution to a group of motors. Such manufacturing variance includes the placing or misplacing of magnets upon the rotor (if surface mounted), variance introduced by the magnetizing process itself and irregularities in the stator coil windings. Other causes of variance include instances when the magnets are damaged or chipped. Further, variations exist even within individual motors. For example, variations typically exist between a motor""s phases and over the motor""s full mechanical cycle. Moreover, motor behavior changes over time as the motor ages.
The phase windings of certain types of electromagnetic machines are energized at least in part as a function of the instantaneous rotor position. Accordingly, such machines often use a rotor position sensor that provides an output indicating rotor position relative to the stator. A controller uses this information to produce control signals that are used to energize and de-energize the phase windings. Errors in the measurement of the angular position of the rotor also contribute to torque ripple.
For many motor applications a slight non-uniformity in the rotation of the rotor caused by torque irregularities is of little or no consequence. For example, in large motors driving large loads, slight variations in the output torque will not significantly affect the rotor speed and any slight variations in rotor speed will not significantly affect the system being driven by the machine. This assumes that the torque variation as the machine turns is small compared to the load. In other applications, where the rotation of the rotor or the torque output of the motor must be precisely controlled or uniform, such non-uniformity is not acceptable. For example, in servomotors used in electric power steering systems and is in disk drives, the rotational output of the rotor or the torque output of the motor must be smooth and without significant variation.
Prior art approaches to reducing the undesirable consequences of torque irregularities in electromagnetic machines have focused on relatively complex rotor or stator constructions designed to eliminate the physical characteristics of the machines that would otherwise give rise to the irregularities. While the prior art machine construction approaches can result in reduction of torque irregularities, the approaches require the design and construction of complex rotor and stator components, such complex components are typically difficult to design, difficult to manufacturer, and much more costly to produce than are conventionally constructed components. Moreover, many of the physical changes required by such prior art solutions result in a significant reduction in the efficiency or other performance parameters of the resulting machines over that expected of comparable conventional machines. Thus, many of the prior art attempts to reduce torque irregularities do so at the cost of machine performance.
Attempts to reduce torque ripple that focus on motor control schemes, rather than motor construction, have also been undertaken. For example, various learning or iterative schemes, based on either experimental procedures or well known physical relationships concerning motor voltages, currents and angular positions have been attempted. These attempted solutions often make assumptions concerning the behavior of the motor, such as motor flux being described by a linear relationship, or considering the effect of mutual flux insignificant. Still further, prior solutions to torque ripple typically ignore the effect of motor sensitivity to inaccuracies in angular measurement. To increase accuracy in position measurement, the use of sophisticated position sensors has been attempted, but this increases the machine""s complexity and cost.
Thus, a need exists for a control system that addresses the shortcomings of the prior art.
In one aspect of the present invention, a system for controlling a rotating electromagnetic machine is presented. The rotating machine, such as a permanent magnet motor or switched reluctance motor, or some hybrid of the two, includes a stator having a plurality of phase windings and a rotor that rotates relative to the stator. A drive is connected to the phase windings for energizing the windings. The control system includes an estimator connectable to the machine for receiving signals representing the phase winding voltage, current and rotor position. The estimator outputs parameter estimations for an electrical model of the machine based on the received voltage, current and rotor position. The electrical model is a mathematical model that describes electrical behavior of the machine as seen at the motor terminals.
A torque model receives the parameter estimations from the estimator. The torque model is developed via a mathematical transform of the electrical model, and describes torque characteristics of the machine. Using the parameters received from the estimator, the torque model estimates torque for associated rotor position-phase current combinations. A controller has input terminals for receiving a torque demand signal and the rotor position signal. The controller outputs a control signal to the drive in response to the torque demand and rotor position signals and the torque model. In certain embodiments, a solver uses the torque model to generate energization current profiles according to desired machine behavior, such as smooth torque and/or minimal sensitivity to errors in rotor position measurement. It is to be noted in particular that the solver can be so defined that the solution possesses particular properties. It may be desired to deal with only the most significant components of cogging or torque ripple, a result of motor drive cost considerations. Such a solution can be achieved.
Some parameters of the torque model are unobservable via information immediately available from the machine terminals. For example, in machines employing permanent magnets, it is not mathematically obvious how changes in the machine current and voltage, as the rotor spins, indicate or measure the interaction of the machine""s magnets with themselves. In accordance with further aspects of the present invention, a method of determining a non-load dependent cogging torque is provided. The rotor is spun unloaded at a predetermined angular velocity and the motor terminal voltage and current are measured. Rotor positions associated with the voltage and current measurements are determined, and a first mathematical model is developed based on the measured voltage and rotor position to describe electrical behavior of the machine. The first mathematical model is mathematically transformed to develop a second mathematical model to describe torque characteristics of the machine. The windings are then energized such that the rotor holds a predetermined position against the cogging torque, and the cogging torque is calculated for the predetermined position via the second mathematical model.